Temat 12: The physics of blood and breathing

Fluid motion is of vital importance. There are at least four fluid circulation systems inside the human body.
First, blood flows through the blood system by the heart.
Second, air is circulated inside the lungs by the diaphragm and other chestmuscles.
Third, lymph flows through the lymphatic vessels, moved passively by body muscles.
Fourth, the cerebrospinal fluid circulates around the brain and the spine, moved by motions of the head.
For this reason, medical doctors like the simple statement: every illness is ultimately due to bad circulation.
Why do living beings have circulation systems?
Circulation is necessary because diffusion is too slow. Can you detail the argument?
We now explore the two main circulation systems in the human body.
Blood keeps us alive: it transports most chemicals required for our metabolism to and from the various parts of our body.
The flow of blood is almost always laminar; turbulence only exists in the venae cavae. The heart pumps around 80 ml of blood per heartbeat, about 5 l/min. At rest, a heartbeat consumes about 1.2 J.
The consumption is sizeable, because the dynamic viscosity of blood ranges between 3.5 ⋅ 10 ⁻³ Pa s (3.5 times higher than water) and 10⁻² Pa s, depending on the diameter of the blood vessel; it is highest in the tiny capillaries.
The speed of the blood is highest in the aorta, where it flows with 0.5 m/s, and lowest in the capillaries, where is as low as 0.3 mm/s. As a result, a substance injected in the arm arrives in the feet between 20 and 60 s after the injection. In fact, all animals have similar blood circulation speeds, usually between 0.2 m/s and 0.4 m/s. Why?
To achieve blood circulation, the heart produces a (systolic) pressure of about 16 kPa, corresponding to a height of about 1.6 m of blood. This value is needed by the heart to pump blood through the brain. When the heart relaxes, the elasticity of the arteries keeps the (diastolic) pressure at around 10 kPa.
These values are measured at the height of the heart. The values vary greatly with the position and body orientation at which they are measured: the systolic pressure at the feet of a standing adult reaches 30 kPa, whereas it is 16 kPa in the feet of a lying person.
For a standing human, the pressure in the veins in the foot is 18 kPa, larger than the systolic pressure in the heart. The high pressure values in the feet and legs is one of the reasons that leads to varicose veins. Nature uses many tricks to avoid problems with blood circulation in the legs.
Humans leg veins have valves to avoid that the blood flows downwards; giraffes have extremely thin legs with strong and tight skin in the legs for the same reason. The same happens for other large animals.
At the end of the capillaries, the pressure is only around 2 kPa. The lowest blood pressure is found in veins that lead back from the head to the heart, where the pressure can even be slightly negative. Because of blood pressure, when a patient receives a (intravenous) infusion, the bag must have a minimum height above the infusion point where the needle enters the body; values of about 0.8 to 1m cause no trouble. (Is the height difference also needed for person-to-person transfusions of blood?)
Since arteries have higher blood pressure, for the more rare arterial infusions, hospitals usually use arterial pumps, to avoid the need for unpractical heights of 2m or more.

The physics of breathing is equally interesting. A human cannot breathe at any depth under water, even if he has a tube going to the surface, as shown in Figure 242 (right):

At a few metres of depth, trying to do so is inevitably fatal!
Even at a depth of 50 cm only, the human body can only breathe in this way for a few minutes, and can get badly hurt for life.Why?
Inside the lungs, the gas exchange with the blood occurs in around 300 millions of little spheres, the alveoli, with a diameter between 0.2 and 0.6 mm. To avoid that the large one grow and the small ones collapse,  the alveoli are covered with a phospholipid surfactant that reduces their surface tension. In newborns, the small radius of the alveoli and the low level of surfactant is the reason that the first breaths, and sometimes also the subsequent ones, require a large effort.
We need around 2% of our energy for breathing alone.The speed of air in the throat is 10 km/h for normal breathing; when coughing, it can be as high as 160 km/h. The flow of air in the bronchi is turbulent; the noise can be heard in a quiet invironment. In normal breathing, the breathingmuscles, in the thorax and in the belly, exchange 0.5 l of air; in a deep breath, the volume can reach 4 l.
Breathing is especially tricky in unusual situations. After scuba diving* at larger
depths than a few meters for more than a few minutes, it is important to rise slowly, to avoid a potentially fatal embolism.Why?
The same can happen to participants in high altitude flights with balloons or aeroplanes, to high altitude parachutists and to cosmonauts

*The blood pressure values measured on the two upper arms also differ; for right handed people, the pressure in the right arm is higher

How blood pressure works


Understanding Blood Pressure
Human Anatomy and Physiology video 3D animation

This is a biology/anatomy video for Grade 10-11 students about Blood Pressure, its causes and effects. The pressure with which blood flows in the blood vessels is called Blood Pressure or BP. BP is measured using a special device called Sphygmomanometer